Search

Using highly improved staggered quark (HISQ) ${N}_{f}=2+1+1$ MILC ensembles with five different values of the lattice spacing, including four ensembles with physical quark masses, we perform the most precise computation to date of the $K\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\ell}\ensuremath{\nu}$ vector form factor at zero momentum transfer, ${f}_{+}^{{K}^{0}{\ensuremath{\pi}}^{\ensuremath{-}}}(0)=0.9696(15{)}_{\mathrm{stat}}(12{)}_{\mathrm{syst}}$. This is the first calculation that includes the dominant finite-volume effects, as calculated in chiral perturbation theory at next-to-leading order. Our result for the form factor provides a direct determination of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $|{V}_{us}|=0.22333(44{)}_{{f}_{+}(0)}(42{)}_{\mathrm{exp}}$, with a theory error that is, for the first time, at the same level as the experimental error. The uncertainty of the semileptonic determination is now similar to that from leptonic decays and the ratio ${f}_{{K}^{+}}/{f}_{{\ensuremath{\pi}}^{+}}$, which uses $|{V}_{ud}|$ as input. Our value of $|{V}_{us}|$ is in tension at the $2--2.6\ensuremath{\sigma}$ level both with the determinations from leptonic decays and with the unitarity of the CKM matrix. In the test of CKM unitarity in the first row, the current limiting factor is the error in $|{V}_{ud}|$, although a recent determination of the nucleus-independent radiative corrections to superallowed nuclear $\ensuremath{\beta}$ decays could reduce the $|{V}_{ud}{|}^{2}$ uncertainty nearly to that of $|{V}_{us}{|}^{2}$. Alternative unitarity tests using only kaon decays, for which improvements in the theory and experimental inputs are likely in the next few years, reveal similar tensions and could be further improved by taking correlations between the theory inputs. As part of our analysis, we calculated the correction to ${f}_{+}^{K\ensuremath{\pi}}(0)$ due to nonequilibrated topological charge at leading order in chiral perturbation theory, for both the full-QCD and the partially quenched cases. We also obtain the combination of low-energy constants in the chiral effective Lagrangian $[{C}_{12}^{r}+{C}_{34}^{r}\ensuremath{-}({L}_{5}^{r}{)}^{2}]({M}_{\ensuremath{\rho}})=(2.92\ifmmode\pm\else\textpm\fi{}0.31)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$.

We introduce a new method for calculating the ${\rm O}(\alpha^3)$ hadronic-vacuum-polarization contribution to the muon anomalous magnetic moment from ${ab-initio}$ lattice QCD. We first derive expressions suitable for computing the higher-order contributions either from the renormalized vacuum polarization function $\hat\Pi(q^2)$, or directly from the lattice vector-current correlator in Euclidean space. We then demonstrate the approach using previously-published results for the Taylor coefficients of $\hat\Pi(q^2)$ that were obtained on four-flavor QCD gauge-field configurations with physical light-quark masses. We obtain $10^{10} a_\mu^{\rm HVP,HO} = -9.3(1.3)$, in agreement with, but with a larger uncertainty than, determinations from $e^+e^- \to {\rm hadrons}$ data plus dispersion relations., Comment: Expanded and clarified discussion and revised Figure 4. Results unchanged. 11 pages, 5 tables, 5 figures. Version accepted to Physical Review D

We calculate in three-flavor lattice QCD the short-distance hadronic matrix elements of all five ΔC=2 four-fermion operators that contribute to neutral D-meson mixing both in and beyond the Standard Model. We use the MILC Collaboration’s Nf=2+1 lattice gauge-field configurations generated with asqtad-improved staggered sea quarks. We also employ the asqtad action for the valence light quarks and use the clover action with the Fermilab interpretation for the charm quark. We analyze a large set of ensembles with pions as light as lattice gauge-field configurations generated with asqtad-improved staggered sea quarks. We also employ the asqtad action for the valence light quarks and use the clover action with the Fermilab interpretation for the charm quark. We analyze a large set of ensembles with pions as light as Mπ ≈ 180 MeV and lattice spacings as fine as a ≈ 0.045 fm, thereby enabling good control over the extrapolation to the physical pion mass and continuum limit. We obtain for the matrix elements in the MS−NDR scheme using the choice of evanescent operators proposed by Beneke et al., evaluated at 3 GeV, ⟨D0|Oi|¯D0⟩ = {0.0805(55)16),−0.1561(70)(31), 0.0464(31)(9), 0.2747(129)(55), 0.1035(71)(21)} GeV4 (i=1–5). The errors shown are from statistics and lattice systematics, and the omission of charmed sea quarks, respectively. To illustrate the utility of our matrix-element results, we place bounds on the scale of CP-violating new physics in D0 mixing, finding lower limits of about 10–50×103 TeV for couplings of O(1). To enable our results to be employed in more sophisticated or model-specific phenomenological studies, we provide the correlations among our matrix-element results. For convenience, we also present numerical results in the other commonly used scheme of Buras, Misiak, and Urban.

We calculate---for the first time in three-flavor lattice QCD---the hadronic matrix elements of all five local operators that contribute to neutral ${B}^{0}$- and ${B}_{s}$-meson mixing in and beyond the Standard Model. We present a complete error budget for each matrix element and also provide the full set of correlations among the matrix elements. We also present the corresponding bag parameters and their correlations, as well as specific combinations of the mixing matrix elements that enter the expression for the neutral $B$-meson width difference. We obtain the most precise determination to date of the SU(3)-breaking ratio $\ensuremath{\xi}=1.206(18)(6)$, where the second error stems from the omission of charm-sea quarks, while the first encompasses all other uncertainties. The threefold reduction in total uncertainty, relative to the 2013 Flavor Lattice Averaging Group results, tightens the constraint from $B$ mixing on the Cabibbo-Kobayashi-Maskawa (CKM) unitarity triangle. Our calculation employs gauge-field ensembles generated by the MILC Collaboration with four lattice spacings and pion masses close to the physical value. We use the asqtad-improved staggered action for the light-valence quarks and the Fermilab method for the bottom quark. We use heavy-light meson chiral perturbation theory modified to include lattice-spacing effects to extrapolate the five matrix elements to the physical point. We combine our results with experimental measurements of the neutral $B$-meson oscillation frequencies to determine the CKM matrix elements $|{V}_{td}|=8.00(34)(8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, $|{V}_{ts}|=39.0(1.2)(0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, and $|{V}_{td}/{V}_{ts}|=0.2052(31)(10)$, which differ from CKM-unitarity expectations by about $2\ensuremath{\sigma}$. These results and others from flavor-changing-neutral currents point towards an emerging tension between weak processes that are mediated at the loop and tree levels.

We compute the form factors for the $B\ensuremath{\rightarrow}K{l}^{+}{l}^{\ensuremath{-}}$ semileptonic decay process in lattice QCD using gauge-field ensembles with $2+1$ flavors of sea quark, generated by the MILC Collaboration. The ensembles span lattice spacings from 0.12 to 0.045 fm and have multiple sea-quark masses to help control the chiral extrapolation. The asqtad improved staggered action is used for the light valence and sea quarks, and the clover action with the Fermilab interpretation is used for the heavy $b$ quark. We present results for the form factors ${f}_{+}({q}^{2})$, ${f}_{0}({q}^{2})$, and ${f}_{T}({q}^{2})$, where ${q}^{2}$ is the momentum transfer, together with a comprehensive examination of systematic errors. Lattice QCD determines the form factors for a limited range of ${q}^{2}$, and we use the model-independent $z$ expansion to cover the whole kinematically allowed range. We present our final form-factor results as coefficients of the $z$ expansion and the correlations between them, where the errors on the coefficients include statistical and all systematic uncertainties. We use this complete description of the form factors to test QCD predictions of the form factors at high and low ${q}^{2}$.

We present the first unquenched lattice-QCD calculation of the hadronic form factors for the exclusive decay $\overline{B}\ensuremath{\rightarrow}D\ensuremath{\ell}\overline{\ensuremath{\nu}}$ at nonzero recoil. We carry out numerical simulations on 14 ensembles of gauge-field configurations generated with $2+1$ flavors of asqtad-improved staggered sea quarks. The ensembles encompass a wide range of lattice spacings (approximately 0.045 to 0.12 fm) and ratios of light (up and down) to strange sea-quark masses ranging from 0.05 to 0.4. For the $b$ and $c$ valence quarks we use improved Wilson fermions with the Fermilab interpretation, while for the light valence quarks we use asqtad-improved staggered fermions. We extrapolate our results to the physical point using rooted staggered heavy-light meson chiral perturbation theory. We then parametrize the form factors and extend them to the full kinematic range using model-independent functions based on analyticity and unitarity. We present our final results for ${f}_{+}({q}^{2})$ and ${f}_{0}({q}^{2})$, including statistical and systematic errors, as coefficients of a series in the variable $z$ and the covariance matrix between these coefficients. We then fit the lattice form-factor data jointly with the experimentally measured differential decay rate from BABAR to determine the CKM matrix element, $|{V}_{cb}|=(39.6\ifmmode\pm\else\textpm\fi{}1.{7}_{\mathrm{QCD}+\mathrm{exp}}\ifmmode\pm\else\textpm\fi{}0.{2}_{\mathrm{QED}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. As a byproduct of the joint fit we obtain the form factors with improved precision at large recoil. Finally, we use them to update our calculation of the ratio $R(D)$ in the Standard Model, which yields $R(D)=0.299(11)$.

We present a lattice-QCD calculation of the $B\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\ell}\ensuremath{\nu}$ semileptonic form factors and a new determination of the CKM matrix element $|{V}_{ub}|$. We use the MILC asqtad ($2+1$)-flavor lattice configurations at four lattice spacings and light-quark masses down to $1/20$ of the physical strange-quark mass. We extrapolate the lattice form factors to the continuum using staggered chiral perturbation theory in the hard-pion and SU(2) limits. We employ a model-independent $z$ parametrization to extrapolate our lattice form factors from large-recoil momentum to the full kinematic range. We introduce a new functional method to propagate information from the chiral-continuum extrapolation to the $z$ expansion. We present our results together with a complete systematic error budget, including a covariance matrix to enable the combination of our form factors with other lattice-QCD and experimental results. To obtain $|{V}_{ub}|$, we simultaneously fit the experimental data for the $B\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\ell}\ensuremath{\nu}$ differential decay rate obtained by the BABAR and Belle collaborations together with our lattice form-factor results. We find $|{V}_{ub}|=(3.72\ifmmode\pm\else\textpm\fi{}0.16)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, where the error is from the combined fit to lattice plus experiments and includes all sources of uncertainty. Our form-factor results bring the QCD error on $|{V}_{ub}|$ to the same level as the experimental error. We also provide results for the $B\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\ell}\ensuremath{\nu}$ vector and scalar form factors obtained from the combined lattice and experiment fit, which are more precisely determined than from our lattice-QCD calculation alone. These results can be used in other phenomenological applications and to test other approaches to QCD.

We calculate the $B$-meson decay constants ${f}_{B}$, ${f}_{{B}_{s}}$, and their ratio in unquenched lattice QCD using domain-wall light quarks and relativistic $b$ quarks. We use gauge-field ensembles generated by the RBC and UKQCD collaborations using the domain-wall fermion action and Iwasaki gauge action with three flavors of light dynamical quarks. We analyze data at two lattice spacings of $a\ensuremath{\approx}0.11$, 0.086 fm with unitary pion masses as light as ${M}_{\ensuremath{\pi}}\ensuremath{\approx}290\text{ }\text{ }\mathrm{MeV}$; this enables us to control the extrapolation to the physical light-quark masses and continuum. For the $b$ quarks we use the anisotropic clover action with the relativistic heavy-quark interpretation, such that discretization errors from the heavy-quark action are of the same size as from the light-quark sector. We renormalize the lattice heavy-light axial-vector current using a mostly nonperturbative method in which we compute the bulk of the matching factor nonperturbatively, with a small correction, that is close to unity, in lattice perturbation theory. We also improve the lattice heavy-light current through $\mathcal{O}({\ensuremath{\alpha}}_{s}a)$. We extrapolate our results to the physical light-quark masses and continuum using SU(2) heavy-meson chiral perturbation theory, and provide a complete systematic error budget. We obtain ${f}_{{B}^{0}}=199.5(12.6)\text{ }\text{ }\mathrm{MeV}$, ${f}_{{B}^{+}}=195.6(14.9)\text{ }\text{ }\mathrm{MeV}$, ${f}_{{B}_{s}}=235.4(12.2)\text{ }\text{ }\mathrm{MeV}$, ${f}_{{B}_{s}}/{f}_{{B}^{0}}=1.197(50)$, and ${f}_{{B}_{s}}/{f}_{{B}^{+}}=1.223(71)$, where the errors are statistical and total systematic added in quadrature. These results are in good agreement with other published results and provide an important independent cross-check of other three-flavor determinations of $B$-meson decay constants using staggered light quarks.

We compute the leptonic decay constants $f_{D^+}$, $f_{D_s}$, and $f_{K^+}$, and the quark-mass ratios $m_c/m_s$ and $m_s/m_l$ in unquenched lattice QCD using the experimentally determined value of $f_{\pi^+}$ for normalization. We use the MILC highly improved staggered quark (HISQ) ensembles with four dynamical quark flavors---up, down, strange, and charm---and with both physical and unphysical values of the light sea-quark masses. The use of physical pions removes the need for a chiral extrapolation, thereby eliminating a significant source of uncertainty in previous calculations. Four different lattice spacings ranging from $a\approx 0.06$ fm to $0.15$ fm are included in the analysis to control the extrapolation to the continuum limit. Our primary results are $f_{D^+} = 212.6(0.4)({}^{+1.0}_{-1.2})\ \mathrm{MeV}$, $f_{D_s} = 249.0(0.3)({}^{+1.1}_{-1.5})\ \mathrm{MeV}$, and $f_{D_s}/f_{D^+} = 1.1712(10)({}^{+29}_{-32})$, where the errors are statistical and total systematic, respectively. The errors on our results for the charm decay constants and their ratio are approximately two to four times smaller than those of the most precise previous lattice calculations. We also obtain $f_{K^+}/f_{\pi^+} = 1.1956(10)({}^{+26}_{-18})$, updating our previous result, and determine the quark-mass ratios $m_s/m_l = 27.35(5)({}^{+10}_{-7})$ and $m_c/m_s = 11.747(19)({}^{+59}_{-43})$. When combined with experimental measurements of the decay rates, our results lead to precise determinations of the CKM matrix elements $|V_{us}| = 0.22487(51) (29)(20)(5)$, $|V_{cd}|=0.217(1) (5)(1)$ and $|V_{cs}|= 1.010(5)(18)(6)$, where the errors are from this calculation of the decay constants, the uncertainty in the experimental decay rates, structure-dependent electromagnetic corrections, and, in the case of $|V_{us}|$, the uncertainty in $|V_{ud}|$, respectively., Comment: v2: minor clarifications and additions; version published in Phys. Rev. D. (73 pages, 26 figures.)

We compute the zero-recoil form factor for the semileptonic decay ${\overline{B}}^{0}\ensuremath{\rightarrow}{D}^{*+}{\ensuremath{\ell}}^{\ensuremath{-}}\overline{\ensuremath{\nu}}$ (and modes related by isospin and charge conjugation) using lattice QCD with three flavors of sea quarks. We use an improved staggered action for the light valence and sea quarks (the MILC asqtad configurations), and the Fermilab action for the heavy quarks. Our calculations incorporate higher statistics, finer lattice spacings, and lighter quark masses than our 2008 work. As a byproduct of tuning the new data set, we obtain the ${D}_{s}$ and ${B}_{s}$ hyperfine splittings with few-MeV accuracy. For the zero-recoil form factor, we obtain $\mathcal{F}(1)=0.906(4)(12)$, where the first error is statistical and the second is the sum in quadrature of all systematic errors. With the latest Heavy Flavor Averaging Group average of experimental results and a cautious treatment of QED effects, we find $|{V}_{cb}|=(39.04\ifmmode\pm\else\textpm\fi{}0.4{9}_{\text{expt}}\ifmmode\pm\else\textpm\fi{}0.5{3}_{\mathrm{QCD}}\ifmmode\pm\else\textpm\fi{}0.1{9}_{\mathrm{QED}})\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}3}$. The QCD error is now commensurate with the experimental error.

Using staggered fermions and partially twisted boundary conditions, we calculate the K meson semileptonic decay vector form factor at zero momentum transfer. The HISQ formulation is used for the valence quarks, while the sea quarks are simulated with the asqtad action (MILC Nf = 2 + 1 configurations). For the chiral and continuum extrapolation we use two-loop continuum χPT, supplemented by partially quenched staggered χPT at one loop. Our result is f Kπ + (0) = 0.9667 ±

We study $SU(3)$-breaking effects in the neutral ${B}_{d}$-${\overline{B}}_{d}$ and ${B}_{s}$-${\overline{B}}_{s}$ systems with unquenched ${N}_{f}=2+1$ lattice quantum chromodynamics (QCD). We calculate the relevant matrix elements on the MILC collaboration's gauge configurations with asqtad-improved staggered sea quarks. For the valence light-quarks ($u$, $d$, and $s$) we use the asqtad action, while for $b$ quarks we use the Fermilab action. We obtain $\ensuremath{\xi}={f}_{{B}_{s}}\sqrt{{B}_{{B}_{s}}}/{f}_{{B}_{d}}\sqrt{{B}_{{B}_{d}}}=1.268\ifmmode\pm\else\textpm\fi{}0.063$. We also present results for the ratio of bag parameters ${B}_{{B}_{s}}/{B}_{{B}_{d}}$ and the ratio of Cabibbo-Kobayashi-Maskawa matrix elements $|{V}_{td}|/|{V}_{ts}|$. Although we focus on the calculation of $\ensuremath{\xi}$, the strategy and techniques described here will be employed in future extended studies of the $B$ mixing parameters $\ensuremath{\Delta}{M}_{d,s}$ and $\ensuremath{\Delta}{\ensuremath{\Gamma}}_{d,s}$ in the standard model and beyond.

We calculate form-factor ratios between the semileptonic decays {bar B}{sup 0} {yields} D{sup +} {ell}{sup -}{bar {nu}} and {bar B}{sub s}{sup 0} {yields} D{sub s}{sup +}{ell}{sup -}{bar {nu}} with lattice QCD. These ratios are a key theoretical input in a new strategy to determine the fragmentation fractions of the neutral B decays, which are needed for measurements of BR(B{sub s}{sup 0} {yields} {mu}{sup +}{mu}{sup -}). They use the MILC ensembles of gauge configurations with 2 + 1 flavors of sea quarks at two lattice spacings of approximately 0.12 fm and 0.09 fm. We use the model-independent z parametrization to extrapolate their simulation results at small recoil toward maximum recoil. The results for the form-factor ratios are {line_integral}{sub 0}{sup (s)} (M{sub {pi}}{sup 2})/{line_integral}{sub 0}{sup (d)} (M{sub K}{sup 2}) = 1.046(44){sub stat.}(15){sub syst.} and {line_integral}{sub 0}{sup (s)} (M{sub {pi}}{sup 2})/{line_integral}{sub 0}{sup (d)} (M{sub {pi}}{sup 2}) = 1.054(47){sub stat.}(17){sub syst.}. In contrast to a QCD sum-rule calculation, no significant departure from U-spin (d {leftrightarrow} s) symmetry is observed.

We calculate the leptonic decay constants of B_{(s)} and D_{(s)} mesons in lattice QCD using staggered light quarks and Fermilab bottom and charm quarks. We compute the heavy-light meson correlation functions on the MILC asqtad-improved staggered gauge configurations which include the effects of three light dynamical sea quarks. We simulate with several values of the light valence- and sea-quark masses (down to ~m_s/10) and at three lattice spacings (a ~ 0.15, 0.12, and 0.09 fm) and extrapolate to the physical up and down quark masses and the continuum using expressions derived in heavy-light meson staggered chiral perturbation theory. We renormalize the heavy-light axial current using a mostly nonperturbative method such that only a small correction to unity must be computed in lattice perturbation theory and higher-order terms are expected to be small. We obtain f_{B^+} = 196.9(8.9) MeV, f_{B_s} = 242.0(9.5) MeV, f_{D^+} = 218.9(11.3) MeV, f_{D_s} = 260.1(10.8) MeV, and the SU(3) flavor-breaking ratios f_{B_s}/f_{B} = 1.229(26) and f_{D_s}/f_{D} = 1.188(25), where the numbers in parentheses are the total statistical and systematic uncertainties added in quadrature.

We calculate form-factor ratios between the semileptonic decays ${\overline{B}}^{0}\ensuremath{\rightarrow}{D}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}\overline{\ensuremath{\nu}}$ and ${\overline{B}}_{s}^{0}\ensuremath{\rightarrow}{D}_{s}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}\overline{\ensuremath{\nu}}$ with lattice QCD. These ratios are a key theoretical input in a new strategy to determine the fragmentation fractions of neutral $B$ decays, which are needed for measurements of $\mathrm{BR}({B}_{s}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})$. We use the MILC ensembles of gauge configurations with $2+1$ flavors of sea-quarks at two s of approximately 0.12 fm and 0.09 fm. We use the model-independent $z$ parametrization to extrapolate our simulation results at small recoil toward maximum recoil. Our results for the form-factor ratios are ${f}_{0}^{(s)}({M}_{\ensuremath{\pi}}^{2})/{f}_{0}^{(d)}({M}_{K}^{2})=1.046(44{)}_{\mathrm{stat}}(15{)}_{\mathrm{syst}}$ and ${f}_{0}^{(s)}({M}_{\ensuremath{\pi}}^{2})/{f}_{0}^{(d)}({M}_{\ensuremath{\pi}}^{2})=1.054(47{)}_{\mathrm{stat}}(17{)}_{\mathrm{syst}}$. In contrast to a QCD sum-rule calculation, no significant departure from $U$-spin ($d\ensuremath{\leftrightarrow}s$) symmetry is observed.

We report the non-perturbative tuning of parameters--- kappa_c, kappa_b, and kappa_crit ---that determine the heavy-quark mass in the Fermilab action. This requires the computation of the masses of Ds^(*) and Bs^(*) mesons comprised of a Fermilab heavy quark and a staggered light quark. Additionally, we report the hyperfine splittings for Ds and Bs mesons as a cross-check of our simulation and analysis methods. We find a splitting of 145 +/- 15 MeV for the Ds system and 40 +/- 9 MeV for the Bs system. These are in good agreement with the Particle Data Group average values of 143.9 +/- 0.4 MeV and 46.1 +/- 1.5 MeV, respectively. The calculations are carried out with the MILC 2+1 flavor gauge configurations at three lattice spacings $a$ approximately 0.15, 0.12, and 0.09 fm., 34 pages, 8 figures, 26 tables; some sections rearranged for clarity; conclusions unchanged; version accepted by Phys. Rev.D

We study the lattice spacing dependence, or scaling, of physical quantities using the highly improved staggered quark (HISQ) action introduced by the HPQCD/UKQCD Collaboration, comparing our results to similar simulations with the asqtad fermion action. Results are based on calculations with lattice spacings approximately 0.15, 0.12, and 0.09 fm, using four flavors of dynamical HISQ quarks. The strange and charm quark masses are near their physical values, and the light-quark mass is set to 0.2 times the strange-quark mass. We look at the lattice spacing dependence of hadron masses, pseudoscalar meson decay constants, and the topological susceptibility. In addition to the commonly used determination of the lattice spacing through the static quark potential, we examine a determination proposed by the HPQCD Collaboration that uses the decay constant of a fictitious ''unmixed ss'' pseudoscalar meson. We find that the lattice artifacts in the HISQ simulations are much smaller than those in the asqtad simulations at the same lattice spacings and quark masses.

We demonstrate a method for calculating the neutral B-meson decay constants and mixing matrix elements in unquenched lattice QCD with domain-wall light quarks and static b-quarks. Our computation is performed on the "2+1" flavor gauge configurations generated by the RBC and UKQCD Collaborations with a lattice spacing of a approx 0.11 fm (a^-1 = 1.729 GeV) and a lattice spatial volume of approximately (1.8 fm)^3. We simulate at three different light sea quark masses with pion masses down to approximately 430 MeV, and extrapolate to the physical quark masses using a phenomenologically-motivated fit function based on next-to-leading order heavy-light meson SU(2) chiral perturbation theory. For the b-quarks, we use an improved formulation of the Eichten-Hill action with static link-smearing to increase the signal-to-noise ratio. We also improve the heavy-light axial current used to compute the B-meson decay constant to O(alpha_s p a) using one-loop lattice perturbation theory. We present initial results for the SU(3)-breaking ratios f_{B_s}/f_{B_d} and xi = f_{B_s} sqrt{B_{B_s}}/f_{B_d} sqrt{B_{B_d}}, thereby demonstrating the viability of the method. For the ratio of decay constants, we find f_{B_s}/f_{B_d} = 1.15(12) and for the ratio of mixing matrix elements, we find xi = 1.13(12), where in both cases the errors reflect the combined statistical and systematic uncertainties, including an estimate of the size of neglected O(1/m_b) effects

We calculate the form factor ${f}_{+}({q}^{2})$ for $B$-meson semileptonic decay in unquenched lattice QCD with $2+1$ flavors of light sea quarks. We use Asqtad-improved staggered light quarks and a Fermilab bottom quark on gauge configurations generated by the MILC Collaboration. We simulate with several light-quark masses and at two lattice spacings, and extrapolate to the physical quark mass and continuum limit using heavy-light meson staggered chiral perturbation theory. We then fit the lattice result for ${f}_{+}({q}^{2})$ simultaneously with that measured by the BABAR experiment using a parameterization of the form-factor shape in ${q}^{2}$, which relies only on analyticity and unitarity in order to determine the Cabibbo-Kobayashi-Maskawa matrix element $|{V}_{\mathrm{ub}}|$. This approach reduces the total uncertainty in $|{V}_{\mathrm{ub}}|$ by combining the lattice and experimental information in an optimal, model-independent manner. We find a value of $|{V}_{\mathrm{ub}}|\ifmmode\times\else\texttimes\fi{}{10}^{3}=3.38\ifmmode\pm\else\textpm\fi{}0.36$.

We present the first lattice QCD calculation of the form factor for $\overline{B}\ensuremath{\rightarrow}{D}^{*}\ensuremath{\ell}\overline{\ensuremath{\nu}}$ with three flavors of sea quarks. We use an improved staggered action for the light valence and sea quarks (the MILC configurations), and the Fermilab action for the heavy quarks. The form factor is computed at zero recoil using a new double ratio method that yields the form factor more directly than the previous Fermilab method. Other improvements over the previous calculation include the use of much lighter light-quark masses, and the use of lattice (staggered) chiral perturbation theory in order to control the light-quark discretization errors and chiral extrapolation. We obtain for the form factor, ${\mathcal{F}}_{B\ensuremath{\rightarrow}{D}^{*}}(1)=0.921(13)(20)$, where the first error is statistical and the second is the sum of all systematic errors in quadrature. Applying a 0.7% electromagnetic correction and taking the latest PDG average for ${\mathcal{F}}_{B\ensuremath{\rightarrow}{D}^{*}}(1)|{V}_{cb}|$ leads to $|{V}_{cb}|=(38.7\ifmmode\pm\else\textpm\fi{}{0.9}_{\mathrm{exp} }\ifmmode\pm\else\textpm\fi{}{1.0}_{\mathrm{theo}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$.